The invention relates to a method for determining a steering angle of a motor vehicle and to a device for carrying out the method.
Electrical steering aids are increasingly used in motor vehicles. In order to function, these aids also require, in addition to the mechanical components, such as the steering shaft and gear, electrical and electronic components, such as an actuating device (electric motor), an electronic control unit and sensor technology. In addition to the sensor technology for detecting the torque exerted on the steering wheel by the driver, this technology being required for the basic functioning of a power steering system, sensor technology for detecting the steering position (for example, steering-wheel angle) is required in order to implement additional comfort and safety functions (active damping/active return).
Steering-angle sensors mounted on the steering shaft, such as are used at the present time, above all, in ESP systems, have to detect the angle of rotation of the steering column over a range which is greater than one revolution of the steering shaft. Costly multi-turn sensors are normally used for detecting the absolute position of the steering shaft.
The object on which the invention is based is to specify a method and device for determining the steering angle, by means of which simple and accurate detection of the absolute position of the steering column is possible.
The object is achieved, according to the invention, in that the displacement travel, caused by a steering-wheel deflection, of a rack of a steering gear is detected, the steering angle being determined from said displacement travel, taking into account a predetermined transmission ratio between the steering-wheel angle and the rack travel.
The advantage of steering-angle detection on the rack is that the steering position can be detected unequivocally over the entire steering-wheel deflection angle, whereas, in the case of a steering-angle sensor on the steering column, this measurement task necessitates an additional revolution counter.
The quantities are in a fixed relation via geometrically defined transmission ratios.
This does away with complicated evaluation methods for determining the absolute position. There is no need to use costly multi-turn sensors. Dispensing with mechanical parts, such as revolution counters and the mounting of the steering-angle sensor in engagement, prevents wear, frictional losses and running noises of the transmission mechanism of the multi-turn sensors.
Advantageously, a motor position signal of a drive motor provided for steering assistance is used for validating the steering angle. The motor angle sensor present per se for another system function within the power steering in the motor vehicle is utilized for validating the steering angle. The angular position of the electric motor is also in a predetermined relation via a fixed transmission ratio with the steering-wheel angle, rack travel and vehicle wheel deflection angle. Additional aids for generating redundant signals may therefore be dispensed with.
In one refinement, the steering angle is determined so as to be used for at least one additional steering function, such as active damping and/or steering return.
The steering angle, once determined for the additional steering function, is taken as a basis for determining further vehicle functions. The steering-angle signal is absolute and is available immediately after the system has been cut in, that is to say there is always direct signal detection.
In another refinement of the invention, a device for determining the steering angle of a motor vehicle is provided, in which a steering shaft is connected to a track rod via a steering gear, the position of the steering shaft being detected by means of a position sensor.
According to the invention, the linear position sensor is arranged on the steering gear designed as a rack mechanism, the position of the rack mechanism detected by means of the linear position sensor corresponding to the absolute position of the steering shaft.
The advantage of this invention is that an unequivocal association of the steering angle and the rack is achieved by the measurement of the rack travel, instead of the angle of rotation of the steering column. The rack travel is linked to the steering-wheel angle via an unequivocal transmission ratio.
Complicated evaluation methods for determining the absolute position are thus dispensed with. There is no need to use costly multi-turn sensors.
To determine the steering angle, the position sensor is connected to a control unit which calculates the steering angle and/or the steering speed and/or the steering direction from the displacement travel of the rack of the rack mechanism.
Advantageously, for the validation of the steering-angle signal, the control unit is connected to a motor position sensor arranged on a drive motor which is operatively connected to the steering gear for steering assistance.
In this case, the signal from the travel sensor is validated by the signal of a motor angle sensor required in any event for commutating the drive motor. The motor angle and rack travel are in a proportional relation, so that redundant signal detection by the control unit is possible. This increases the functional reliability of the steering-angle signal to be detected. Sensor technology which is present per se may also be utilized.
In one refinement, the first control unit is connected to at least one further control unit via a data line, via which the steering angle and/or steering speed and/or steering direction determined by the first control unit can be transmitted, the second control unit further processing the steering angle and/or steering speed and/or steering direction for its control and/or regulating functions.
Utilizing existing interfaces of the first control unit, for example the control unit for power steering, makes it possible to dispense with cost-intensive angle sensors for the ESP control unit.
A particularly simple and cost-effective variant is achieved when the linear position sensor for detecting the position of the rack mechanism operates on a non-contact operating principle.
Advantageously, the linear position sensor has a magnetic device and an inductive sensor unit, the magnetic device being firmly connected to the rack of the rack mechanism and the inductive sensor unit being arranged in or on a housing of the rack mechanism, the inductive sensor unit extending over approximately the entire displacement travel of the rack. By virtue of this arrangement, a particularly simple construction of the system is possible.
In a development of the invention, the inductive sensor unit is fastened to the outside of the housing of the steering gear. In this case, the inductive sensor unit detects the position of the permanent magnet through the housing wall of the steering gear housing. The housing consists of a nonferromagnetic material, for example cast aluminum. The sensor is thus an integral component of the power steering.
Advantageously, the magnetic device is detained mechanically on the rack. The magnetic device can, in this case, approximately comprise the circumference of the rack. Alternatively to this, the magnetic device is arranged in a magnet housing which is fastened to the rack by at least one means for positive and/or non-positive fastening.
Particularly stable fastening can be achieved when the rack is bored hollow and the fastening means is arranged so as to engage behind the rack on the inside through a bore of the latter.
A simple manner of production is possible when the magnet housing and the fastening means are formed in one piece as a plastic injection molding.
In a simple refinement, the housing of the rack mechanism has a magnet receptacle formed in the direction of extent of the rack. The magnetic device is guided, free of friction, in this magnet receptacle, within the steering gear housing, along the inductive evaluation unit which emits an electrical signal corresponding to the position of the magnet and to the angle of rotation of the steering column.